'Cop' is an old English word meaning 'to catch'. It comes from the French verb 'caper' and, even further back, from the Latin one 'capere'. That's what a policeman's job is...to catch baddies. Hence the name. Lots of people say it comes from the copper buttons they supposedly had on their uniforms long ago and others say it is an acronym for 'Constable on Patrol'. However, I'd advise you to go with the etymology offered by the word-bible...The Oxford English Dictionary. It says cop = catch is the explanation.